Chapter 2
The Copernican Revolution
Units of Chapter 2
2.1 Ancient Astronomy
2.2 The Geocentric Universe
2.3 The Heliocentric Model of the Solar System
The Foundations of the Copernican Revolution
2.4 The Birth of Modern Astronomy
Units of Chapter 2, continued
2.5 The Laws of Planetary Motion
Some Properties of Planetary Orbits
2.6 The Dimensions of the Solar System
2.7 Newton’s Laws
The Moon is Falling!
2.8 Newtonian Mechanics
Weighing the Sun
2.1 Ancient Astronomy
• Ancient civilizations observed the skies
• Many built structures to mark astronomical
events
Summer solstice
sunrise at
Stonehenge:
2.1 Ancient Astronomy
Spokes of the Big Horn Medicine Wheel are
aligned with rising and setting of Sun and
other stars
2.1 Ancient Astronomy
This temple at
Caracol, in Mexico,
has many windows
that are aligned with
astronomical events
2.2 The Geocentric Universe
Ancient astronomers
observed:
Sun
Moon
Stars
Five planets: Mercury,
Venus, Mars, Jupiter,
Saturn
2.2 The Geocentric Universe
Sun, Moon, and stars all have simple
movements in the sky
Planets:
• Move with respect to
fixed stars
• Change in brightness
• Change speed
• Undergo retrograde
motion
2.2 The Geocentric Universe
• Inferior planets: Mercury, Venus
• Superior planets: Mars, Jupiter, Saturn
Now know:
Inferior planets have
orbits closer to Sun
than Earth’s
Superior planets’
orbits are farther
away
2.2 The Geocentric Universe
Early observations:
• Inferior planets never too far from Sun
• Superior planets not tied to Sun; exhibit
retrograde motion
• Superior planets brightest at opposition
• Inferior planets brightest near inferior
conjunction
2.2 The Geocentric Universe
Earliest models had Earth at center of solar
system
Needed lots of
complications to
accurately track
planetary motions
2.3 The Heliocentric Model of the
Solar System
Sun is at center of solar system. Only Moon
orbits around Earth; planets orbit around Sun.
This figure
shows
retrograde
motion of
Mars.
Discovery 2-1: The Foundations of
the Copernican Revolution
1. Earth is not at the center of everything.
2. Center of earth is the center of moon’s orbit.
3. All planets revolve around the Sun.
4. The stars are very much farther away than the
Sun.
5. The apparent movement of the stars around the
Earth is due to the Earth’s rotation.
6. The apparent movement of the Sun around the
Earth is due to the Earth’s rotation.
7. Retrograde motion of planets is due to Earth’s
motion around the Sun.
2.4 The Birth of Modern Astronomy
Telescope invented around
1600
Galileo built his own, made
observations:
• Moon has mountains and
valleys
• Sun has sunspots, and
rotates
• Jupiter has moons (shown):
• Venus has phases
2.4 The Birth of Modern Astronomy
Phases of
Venus cannot
be explained by
geocentric
model
2.5 The Laws of Planetary Motion
Kepler’s laws were
derived using
observations made by
Tycho Brahe
2.5 The Laws of Planetary Motion
1. Planetary orbits are ellipses, Sun at one focus
2.5 The Laws of Planetary Motion
2. Imaginary line connecting Sun and planet
sweeps out equal areas in equal times
2.5 The Laws of Planetary Motion
3. Square of period of planet’s orbital motion
is proportional to cube of semimajor axis
More Precisely 2-1: Some
Properties of Planetary Orbits
Semimajor axis and eccentricity of orbit
completely describe it
Perihelion: closest approach to Sun
Aphelion: farthest distance from Sun
2.6 The Dimensions of the Solar System
Astronomical unit: mean distance from
Earth to Sun
First measured during transits of Mercury
and Venus, using triangulation
2.6 The Dimensions of the Solar System
Now measured using radar:
Ratio of mean
radius of Venus’s
orbit to that of
Earth very well
known
2.7 Newton’s Laws
Newton’s laws of motion
explain how objects
interact with the world
and with each other.
2.7 Newton’s Laws
Newton’s First Law:
An object at rest will remain at rest, and an object
moving in a straight line at constant speed will
not change its motion, unless an external force
acts on it.
2.7 Newton’s Laws
Newton’s second law:
When a force is exerted on an object, its
acceleration is inversely proportional to its mass:
a = F/m
Newton’s third law:
When object A exerts a force on object B, object
B exerts an equal and opposite force on object A.
2.7 Newton’s Laws
Gravity
On the Earth’s
surface, acceleration
of gravity is
approximately
constant, and
directed toward the
center of Earth
2.7 Newton’s Laws
Gravity
For two massive
objects, gravitational
force is proportional to
the product of their
masses divided by the
square of the distance
between them
2.7 Newton’s Laws
Gravity
The constant G is called the gravitational
constant; it is measured experimentally and
found to be:
G = 6.67 x 10-11 N m2/kg2
More Precisely 2-2: The Moon is Falling!
Newton’s insight: same force causes apple to
fall and keeps Moon in orbit; decreases as
square of distance, as does centripetal
acceleration: a = v2/r
2.8 Newtonian Mechanics
Kepler’s laws are a
consequence of
Newton’s laws; first
law needs to be
modified: The orbit of
a planet around the
Sun is an ellipse, with
the center of mass of
the planet–Sun
system at one focus.
More Precisely 2-3: Weighing the Sun
Newtonian mechanics tells us that the force
keeping the planets in orbit around the Sun is the
gravitational force due to the masses of the
planet and Sun.
This allows us to calculate the mass of the Sun,
knowing the orbit of the Earth:
M = rv2/G
The result is M = 2.0 x 1030 kg (!)
2.8 Newtonian Mechanics
Escape speed: the
speed necessary for
a projectile to
completely escape a
planet’s gravitational
field. With a lesser
speed, the projectile
either returns to the
planet or stays in
orbit.
Summary of Chapter 2
First models of solar system were geocentric
but couldn't easily explain retrograde motion
Heliocentric model does; also explains
brightness variations
Galileo's observations supported heliocentric
model
Kepler found three empirical laws of
planetary motion from observations
Summary of Chapter 2, continued
Laws of Newtonian mechanics explained
Kepler’s observations.
Gravitational force between two masses is
proportional to the product of the masses,
divided by the square of the distance
between them.